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      <h1 id="Java-基础-泛型擦除"><a href="#Java-基础-泛型擦除" class="headerlink" title="Java-基础-泛型擦除"></a>Java-基础-泛型擦除</h1><h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>[参考博客][<a href="https://www.cnblogs.com/wuqinglong/p/9456193.html]" target="_blank" rel="noopener">https://www.cnblogs.com/wuqinglong/p/9456193.html]</a></p>
<ul>
<li>大家都知道，Java的泛型是伪泛型，<strong>这是因为Java在编译期间，所有的泛型信息都会被擦掉</strong>，正确理解泛型概念的首要前提是理解类型擦除。Java的泛型基本上都是在编译器这个层次上实现的，在生成的字节码中是不包含泛型中的类型信息的，<strong>使用泛型的时候加上类型参数，在编译器编译的时候会去掉，这个过程成为类型擦除。</strong></li>
<li><strong>如在代码中定义<code>List&lt;Object&gt;</code>和<code>List&lt;String&gt;</code>等类型，在编译后都会变成<code>List</code></strong>，JVM看到的只是List，而由泛型附加的类型信息对JVM是看不到的。Java编译器会在编译时尽可能的发现可能出错的地方，<strong>但是仍然无法在运行时刻出现的类型转换异常的情况</strong>，类型擦除也是Java的泛型与C++模板机制实现方式之间的重要区别。</li>
</ul>
<a id="more"></a>



<h2 id="1-通过两个例子证明Java类型的类型擦除"><a href="#1-通过两个例子证明Java类型的类型擦除" class="headerlink" title="1. 通过两个例子证明Java类型的类型擦除"></a>1. 通过两个例子证明Java类型的类型擦除</h2><ol>
<li><h4 id="原始类型相等"><a href="#原始类型相等" class="headerlink" title="原始类型相等"></a>原始类型相等</h4></li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">        ArrayList&lt;String&gt; list1 = <span class="keyword">new</span> ArrayList&lt;String&gt;();</span><br><span class="line">        list1.add(<span class="string">"abc"</span>);</span><br><span class="line"></span><br><span class="line">        ArrayList&lt;Integer&gt; list2 = <span class="keyword">new</span> ArrayList&lt;Integer&gt;();</span><br><span class="line">        list2.add(<span class="number">123</span>);</span><br><span class="line"></span><br><span class="line">        System.out.println(list1.getClass() == list2.getClass());</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在这个例子中，我们定义了两个<code>ArrayList</code>数组，不过一个是<code>ArrayList&lt;String&gt;</code>泛型类型的，只能存储字符串；一个是<code>ArrayList&lt;Integer&gt;</code>泛型类型的，只能存储整数，最后，我们通过<code>list1</code>对象和<code>list2</code>对象的<code>getClass()</code>方法获取他们的类的信息，最后发现结果为<code>true</code>。说明泛型类型<code>String</code>和<code>Integer</code>都被擦除掉了，只剩下原始类型。</p>
<ol start="2">
<li><h4 id="通过反射添加其它类型元素"><a href="#通过反射添加其它类型元素" class="headerlink" title="通过反射添加其它类型元素"></a>通过反射添加其它类型元素</h4></li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> Exception </span>&#123;</span><br><span class="line"></span><br><span class="line">        ArrayList&lt;Integer&gt; list = <span class="keyword">new</span> ArrayList&lt;Integer&gt;();</span><br><span class="line"></span><br><span class="line">        list.add(<span class="number">1</span>);  <span class="comment">//这样调用 add 方法只能存储整形，因为泛型类型的实例为 Integer</span></span><br><span class="line"></span><br><span class="line">        list.getClass().getMethod("add", Object.class).invoke(list, "asd");</span><br><span class="line"></span><br><span class="line">        <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; list.size(); i++) &#123;</span><br><span class="line">            System.out.println(list.get(i));</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在程序中定义了一个<code>ArrayList</code>泛型类型实例化为<code>Integer</code>对象，如果直接调用<code>add()</code>方法，那么只能存储整数数据，不过当我们利用反射调用<code>add()</code>方法的时候，却可以存储字符串，这说明了<code>Integer</code>泛型实例在编译之后被擦除掉了，只保留了原始类型。</p>
<h2 id="2-类型擦除后保留的原始类型"><a href="#2-类型擦除后保留的原始类型" class="headerlink" title="2. 类型擦除后保留的原始类型"></a>2. 类型擦除后保留的原始类型</h2><ul>
<li><p>在上面，两次提到了原始类型，什么是原始类型？</p>
<p><strong>原始类型</strong> 就是擦除去了泛型信息，最后在字节码中的类型变量的真正类型，无论何时定义一个泛型，相应的原始类型都会被自动提供，类型变量擦除，并使用其限定类型（无限定的变量用Object）替换。</p>
</li>
</ul>
<p><strong>例3.原始类型Object</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Pair</span>&lt;<span class="title">T</span>&gt; </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> T value;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> T <span class="title">getValue</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> value;  </span><br><span class="line">    &#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setValue</span><span class="params">(T  value)</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">this</span>.value = value;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>Pair的原始类型为:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Pair</span> </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> Object value;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> Object <span class="title">getValue</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> value;  </span><br><span class="line">    &#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setValue</span><span class="params">(Object  value)</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">this</span>.value = value;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>因为在<code>Pair&lt;T&gt;</code>中，T 是一个无限定的类型变量，所以用<code>Object</code>替换，其结果就是一个普通的类，如同泛型加入Java语言之前的已经实现的样子。在程序中可以包含不同类型的<code>Pair</code>，如<code>Pair&lt;String&gt;</code>或<code>Pair&lt;Integer&gt;</code>，但是擦除类型后他们的就成为原始的<code>Pair</code>类型了，原始类型都是<code>Object</code>。</p>
<p><strong>从上面的例2中，我们也可以明白<code>ArrayList&lt;Integer&gt;</code>被擦除类型后，原始类型也变为<code>Object</code>，所以通过反射我们就可以存储字符串了。</strong></p>
<p>如果类型变量有限定，那么原始类型就用第一个边界的类型变量类替换。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Pair</span>&lt;<span class="title">T</span> <span class="keyword">extends</span> <span class="title">Comparable</span>&gt; </span>&#123;&#125;</span><br></pre></td></tr></table></figure>

<p>那么原始类型就是<code>Comparable</code>。</p>
<p>要区分原始类型和泛型变量的类型。</p>
<p>在调用泛型方法时，可以指定泛型，也可以不指定泛型。</p>
<ul>
<li>在不指定泛型的情况下，泛型变量的类型为该方法中的几种类型的同一父类的最小级，直到Object</li>
<li>在指定泛型的情况下，该方法的几种类型必须是该泛型的实例的类型或者其子类</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>&#123;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;  </span><br><span class="line"></span><br><span class="line">        <span class="comment">/**不指定泛型的时候*/</span>  </span><br><span class="line">        <span class="keyword">int</span> i = Test.add(<span class="number">1</span>, <span class="number">2</span>); <span class="comment">//这两个参数都是Integer，所以T为Integer类型  </span></span><br><span class="line">        Number f = Test.add(<span class="number">1</span>, <span class="number">1.2</span>); <span class="comment">//这两个参数一个是Integer，以风格是Float，所以取同一父类的最小级，为Number  </span></span><br><span class="line">        Object o = Test.add(<span class="number">1</span>, <span class="string">"asd"</span>); <span class="comment">//这两个参数一个是Integer，以风格是Float，所以取同一父类的最小级，为Object  </span></span><br><span class="line"></span><br><span class="line">        <span class="comment">/**指定泛型的时候*/</span>  </span><br><span class="line">        <span class="keyword">int</span> a = Test.&lt;Integer&gt;add(<span class="number">1</span>, <span class="number">2</span>); <span class="comment">//指定了Integer，所以只能为Integer类型或者其子类  </span></span><br><span class="line">        <span class="keyword">int</span> b = Test.&lt;Integer&gt;add(<span class="number">1</span>, <span class="number">2.2</span>); <span class="comment">//编译错误，指定了Integer，不能为Float  </span></span><br><span class="line">        Number c = Test.&lt;Number&gt;add(<span class="number">1</span>, <span class="number">2.2</span>); <span class="comment">//指定为Number，所以可以为Integer和Float  </span></span><br><span class="line">    &#125;  </span><br><span class="line"></span><br><span class="line">    <span class="comment">//这是一个简单的泛型方法  </span></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> &lt;T&gt; <span class="function">T <span class="title">add</span><span class="params">(T x,T y)</span></span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> y;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<p><strong>例4.Object泛型</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;  </span><br><span class="line">    ArrayList list = <span class="keyword">new</span> ArrayList();  </span><br><span class="line">    list.add(<span class="number">1</span>);  </span><br><span class="line">    list.add(<span class="string">"121"</span>);  </span><br><span class="line">    list.add(<span class="keyword">new</span> Date());  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<h2 id="3-带来的问题和解决方法"><a href="#3-带来的问题和解决方法" class="headerlink" title="3. 带来的问题和解决方法"></a>3. 带来的问题和解决方法</h2><p>因为种种原因，Java不能实现真正的泛型，只能使用类型擦除来实现伪泛型，这样虽然不会有类型膨胀问题，但是也引起来许多新问题，所以，SUN对这些问题做出了种种限制，避免我们发生各种错误。</p>

      
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